Wavelength division multiplexing (WDM) has been used to increase the capacity of existing fiber optic networks. In these systems, a plurality of optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength.
A WDM device can be used to multiplex the plurality of optical signal channels into a single optical signal (e.g., a multiplexer), or demultiplex a single optical signal corresponding to a plurality of optical signal channels into the individual channels (e.g., a demultiplexer). Typically, state-of-the art WDM devices are based on thin-film filters (TFFs), wherein standard deposition processes are used to create narrow-band transmission filters, one for each channel to be multiplexed. The specifications of these unpackaged filters depend on the technology used to create them, however, a reflection loss of 0.2 dB is typical.
FIG. 1 illustrates one embodiment of a prior art multiplexing device using TFFs. The multiplexing device includes a single TFF and two gradient index (GRIN) lenses packaged as a three-port device. Each port of the multiplexing device is coupled to a separate optical fibre, namely, the input port, the upgrade, and the add/drop port. Due to the inherent aberrations of the GRIN lenses, the loss of this device is roughly 0.3 dB greater than the unpackaged filter.
To create a WDM module (i.e., a device which has more than one add/drop channel) with this technology, a set of three-port devices, one for each wavelength to be dropped or added, are made. These three-port devices are then cascaded with fiber connections to create a module. Typical specifications for such modules are given in Table 1.
TABLE 1Typical specifications for thin film filter WDMs.Channel Count816Channel Spacing[GHz]100100Insertion Loss[dB]46Insertion Loss Uniformity[dB]1.52Adj. Channel Crosstalk[dB]−25−25Non-Adj. Channel Crosstalk[dB]−40−40Passband Ripple[dB]11PDL[dB]0.10.1Return Loss[dB]4040
Table 1 shows that the insertion loss (IL) of the WDM module is significantly higher than the sum of the bare filter losses; this is the penalty for entering and exiting all of the discrete 3-port packages. Similarly, the size, cost, and assembly time are quite high.
Accordingly, more recent development has been aimed at packaging the same TFFs in free-space, rather than with fiber interconnections. For example, see U.S. Pat. No. 5,859,717 to MA. Scobey et al., and/or UK Pat. Appl. GB 2014752 to K. Hashimoto et al., herein incorporated by reference. The basic idea is to route the optical signal along a zigzag path that is formed by two parallel rows of TFFs as schematically shown in FIG. 2. These free-space interconnects can in principle be lower in loss, smaller, and more quickly constructed than the traditional WDM module.
However, the commercialization of such free-space packaging of TFFs has been generally hindered by some technical difficulties. In particular, beam diffraction and the robustness of optical alignment have been major concerns.
For example, for optimum performance and ease of fabrication, TFFs may be used near normal incidence—typically 1.8 degrees to minimize the polarization dependence of the filter transmission. In order to physically separate the incident beams on adjacent filters via free-space propagation, the package length d between the filter rows must exceed half the filter width divided by the tangent of the incidence angle on the filters. For a typical filter size of ˜1.5 mm, d becomes nearly 30 mm. If eight TFFs are cascaded, the total propagation length from input port to the last port becomes about 30×(8+1)=270 mm. Such a long propagation length will lead to diffraction of the optical beam and thus an increase of the beam diameter. For example, a Gaussian beam that has a diameter of 400 μm at its beam waist will have a diameter of about 780 μm at the propagation length of +/−135 mm from the beam waist. Such an increase in beam diameter makes it difficult to efficiently couple light back into lenses and then into fibers, leading to an excess insertion loss of the device. Accordingly, add/drop ports are generally limited to a relatively small number (e.g., four or less). Additionally, the long propagation length causes the optical alignment to be sensitive to a change in the incident beam angle. Therefore, the alignment robustness and reliability becomes another serious issue.